Joekd, They are probably almost as bright straight on, but the total lumen output is only a fraction. Go off to the sides and they aren't as bright. 3157 clear bulbs put out about 300-400 lumens. After going through the red lens of the tail-light they are about 100-150 lumens. Each LED in one of those retrofit bulbs put out less than one lumen per LED, multiplied by 15 and the most you can get is 15 lumens... and that's and that is absolute best case scenario. As a reference, the LEDs I have put out 3 lumens per LED under perfect conditions... so I'm guessing 2.5 lumens. I have 40 for my tail-lights (100 lumens) and another 114 for my brake lights (285 lumens). Since they are all on with my brake lights, total output is 385 lumens, then take away about 15-25% for the lens, so worse case scenario, mathematically, I have about 288.75 lumens per tail-light when the brakes are on.
I'm not saying that the retrofit bulbs suck... they're cool. I love them for my front blinkers since there are no secondary optics to diffuse the light. I just wouldn't use them as my brake lights since I'm anal about people being able to notice when I have my brake lights on. For those that aren't so paranoid about their brake lights being seen as well as the sun... the retrofit bulbs are great. They're also a hell of a lot easier to install!
Anyways, that's my opinion... I want my brake lights to be very bright... at least if not brighter than stock brake lights.